We use the type of mat Pawz4me linked to. Ours is 9' by 18'. If we want it waterproof (from the bottom) we put down a polyethylene tarp first. Unfortunately, the mat makes and tarp makers can't seem to agree on a standard size. The tarp we (occasionally) use is 12' by 20'.
We use a mat for our patio area. We also use a mat if we are setting up x-pens for the dogs. (Plural pens, we will use at least two, because we travel with 4 dogs.) It is nice to use the x-pens and mats after the dogs have been swimming.
Tom